1. Field of the Invention
The present invention relates to an optical disc device for converting, for example, a high quality television signal to an image signal for a MUSE (Multiple Sub-Nyquist Sampling Encoding) system and recording and playing back the image signal.
2. Description of Related Art
In conventional optical disc devices for recording and playing back image signals for MUSE systems, the time base of the playback signal can be corrected based on this pilot signal by frequency multiplexing and then recording the pilot signal and deterioration of image quality can be effectively prevented. FIG. 7 shows the whole of the optical disc device, with high quality television signals being recorded onto a so-called "write once" optical disc 2.
To form this optical disc 2, a polycarbonate board is spin coated with an organic pigment film. Then, a reflecting film is formed by vapor deposition, with this then being coated in a protective plastic layer. Further, a so-called "pregroove" which acts as a guide channel for a laser beam is formed so as to wind through the information recording surface. Tracking control may then be achieved by using this pregroove and position information for the position of illumination of the laser beam may be detected.
Namely, with regards to the optical disc device 1, the laser beam emitted from the internal laser diode of the optical pick-up 3 is focused onto the information recording surface of the optical disc 2 by an internal objective lens. The light returned from the optical disc 2 is then focused by the objective lens and received by an internal photoelectric detector. The receiving surface of the photoelectric detector is divided, for example, across the direction of the radius and the circumference of the optical disc 2, with output signals being outputted for each of the divided light-receiving surfaces.
Moreover, with regards to the optical disc 1, after current/voltage conversion in a current/voltage converter not shown in the drawings, the output signals of each of the receiving surfaces are amplified by a preamplifier 4 (PRE AMP). The servo circuit 5 performs arithmetic processes on the output signal of the preamplifier 4 and, as a result of this, generates a tracking error signal and a focus error signal. The servo circuit 5 adjusts and object lens of the optical pick-up 3 based on the tracking error and focus error signals so as to form an overall servo loop in the optical disc device 1, and tracking control and focus control may therefore be exerted.
With regards to a position detector circuit 6, after the output signal of a current/voltage converter circuit is amplified at a prescribed gain, arithmetic processes are carried out. In this way, a wobble signal, the signal level of which varies in response to the windings of the pregroove, is generated. A spindle servo circuit 7 starts operating under the control of a system controller circuit 8 and rotatably drives the spindle motor 9 so that the carrier frequency of the wobble signal becomes a prescribed frequency. In this way, a servo loop is formed and the optical disc 2 is rotated under constant linear velocity conditions.
Further, the position detector circuit 6 detects position information for the laser beam illumination position by demodulating this wobble signal using a built-in FM (Frequency Modulation) demodulation circuit and the detection results are outputted to the system controller circuit 8. In this way, at the optical disc device 1, overall operation is controlled at the system controller circuit 8 based on this position information so that a high quality television signal may be recorded at or played back from a prescribed region.
This is to say that the optical disc device 1 rotatably drives the optical disc 2 under constant linear velocity conditions. Further, the high quality television signal S1 outputted from the video cassette recorder (VCR) 10 is inputted to a MUSE encoder (ENC) 11 together with an audio signal. The MUSE encoder 11 then converts the high quality television signal S1 and the audio signal into a MUSE system image signal S2 and outputs the result. Moreover, the MUSE encoder 11 generates a demodulation reference signal S3 for the image signal S2 and this reference signal S3 is outputted to a pilot generator (GEN) 12.
The pilot generator (GEN) 12 generates a sine wave pilot signal PL with a frequency of 2.278125 MHz based on the reference signal S3 and this pilot signal PL is recorded on the optical disc 2 as a time-base corrected reference signal. The address encoder (ENC) 13 then generates an address signal consisting of a time code signal for the image signal S2 provided from the VCR 10, which is then outputted to a mixer 14 at a prescribed time. In this way, the mixer 14 time-base multiplexes this address signal at the 564 lines of the image signal S2 without restrictions with regards to the user area.
The frequency modulation circuit (FM MOD) 16 modulates the image signal S2 outputted from the mixer 14 at a central frequency of 12.5 MHz. The mixer 17 adds the pilot signal PL to the output signal S4 of the frequency modulation circuit 16 and outputs the result. The mixer 17 also frequency multiplexes an audio signal SD of a prescribed format. As shown in FIG. 8, with regards to the optical disc device 1, the frequency modulated signal S4 of the image signal S2, the pilot signal PL and the digital audio signal SD are frequency multiplexed and then recorded on the optical disc 2.
Namely, a laser diode modulation circuit (LD MOD) 18 drives the internal laser diode of the optical pick-up 3 and increases the amount of laser beam light from the amount used during playback to the amount required during recording on the rising edge of the output signal from the mixer 17. In this way, at the optical disc device 1, the amount of laser beam light intermittently rises in response to the output signal for the mixer 17, a sequence of pits are formed on the optical disc 2 and a high quality television signal S1 is recorded.
With respect to this, and referring to FIG. 9, during playback, the optical disc device 1 successively projects a laser beam from the optical pick-up 3 using the amount of light at the time of playback and an output signal for the optical pick-up which can be obtained from these results is outputted to the preamplifier 4. In this way, at the optical disc device 1, the output signal for the preamplifier 4 is processed using the servo circuit 5 in the same way as at the time of recording so as to control the tracking and the focus. Further, at the time of playback, the preamplifier 4 generates a playback signal for which the signal level varies in response to the amount of returned light by adding the output signals for each of the light receiving surfaces. This playback signal is then band limited and outputted.
This band limiting separates each of the signals PL, S4 and SD in accordance with the frequency allocation illustrated in FIG. 8. At the optical disc device 1, the playback signal for the pilot signal of these signals is outputted to the PLL (Phase Locked Loop) circuit 20. The PLL circuit 20 then generates a reference signal taking the playback signal for this pilot signal PL as a reference and a dividing signal S6 for this reference signal is then outputted to the spindle servo circuit 7.
In place of a wobble signal, at the time of playback, the spindle servo circuit 7 drives the spindle motor 9 so that the frequency of this divided frequency S6 becomes a predetermined frequency. As a result of this, with the write-once-type optical disc replaced, the optical disc can be rotatably driven under constant linear velocity conditions at the optical disc device 1 even when playing back from a playback-dedicated optical disc made by, for example, sputtering. At this time, the spindle servo circuit 7 drives the spindle motor 9 taking the internal clock CK generated by a clock generating circuit 21 as a reference.
Further, the PLL circuit 20 generates a 27.3375 MHz clock signal CK1 from the reference signal and the playback signal is then time-base corrected at the optical disc 1 based on this clock signal CK1. The address decoder (DEC) 23 extracts and plays back the time-base multiplexed address signal by capturing and demodulating using an internal demodulating circuit the playback signal RF outputted from the preamplifier 4 at a prescribed timing. The playback results are then outputted to the system controller circuit 8. In this way, the optical disc 2 may be played back at the optical disc 1 based on this address information.
A frequency demodulation circuit (FM DEMOD) 24 modulates the playback signal RF and plays back a MUSE system image signal S7, with this played back image signal S7 being outputted to a time base correction circuit 25. The time base correction circuit 25 stores this image signal S7 taking the clock signal CK1 outputted from the PLL circuit 20 as a reference and outputs the stored image signal S7 taking the internal clock CK2 outputted from the clock generating circuit 21 as a reference so that the image signal S7 can be time base corrected.
The muse decoder (MUSE DEC) 26 converts the image signal S7 from the TBC 25 to a high-quality television signal S8 and then outputs this signal, which is the opposite to the time of recording. The image signal recorded on the optical disc 2 may then be monitored at the optical disc device 1 via a monitor, etc.
This kind of optical disc device 1 illuminates the optical disc 2 with a laser beam, alters the temperature of the information recording surface in a localized manner and forms a sequence of pits. The optical disc device 1 is therefore characterized by changes in the size of the pits formed on the optical disc 2 in response to the surrounding temperature, the optical disc sensitivity and the amount of laser beam light, etc. The asymmetry of the playback signal RF may be changed greatly by the conditions at the time of recording. Because of this, the optical disc device 1 is characterized by the duty of the RF playback signal being changed greatly by the conditions at the time of recording.
When the duty of the RF playback signal is changed in this way, the RF playback signal incurs non-linear distortion with respect to the laser diode driving signal (i.e. the signal inputted to the laser diode modulation circuit 18 of FIG. 8) at the time of recording and finally, intermodulation distortion is generated. As a result of this, the picture quality for the played-back image signal S7 deteriorates due to intermodulation distortion. Therefore, with this kind of optical disc device 1, the amount of laser beam light has to be controlled severely in response to the surrounding temperature, which complicates the overall construction.
In order to resolve these problems, one method was considered where a pilot signal PL (see FIG. 8) for pre-time base correction was recorded. i.e. a pilot signal PL was pre-recorded on the disc by preformatting the optical disc 2. Then, when recording, the image signal S7 was recorded using the pilot signal PL as a reference. However, in the case of this method, the image signal S7 had to be time base-corrected and then recorded at the time of recording taking the playback results for the pilot signal PL as a reference in order to be in synchronization with this pilot signal PL. This made the overall structure complicated.
As the present invention sets out to resolve the above problems, it is an object of the present invention to provide an optical disc device for converting a high-quality television signal to a MUSE system image signal and recording and playing back the MUSE system image signal, capable of preventing the playback signal from incurring interference from intermodulation distortion in a simple manner.